At present, a wide variety of IMDs are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogatable using RF telemetry transmissions. Such IMDs include implantable cardiac pacemakers, cardioverter/defibril lators, cardiomyostimulators, pacemaker/cardioverter/defibrillators, drug delivery systems, cardiac and other physiologic monitors, electrical stimulators including nerve and muscle stimulators, deep brain stimulators, cochlear implants, and heart assist devices or pumps, etc.
Most such IMDs comprise electronic circuitry and an IMD battery that provides power to the electronic circuitry and that depletes in energy over time. Therefore, it is necessary to monitor the state of the battery in such IMDs so that the IMD can be replaced before the battery depletes to a state that renders the IMD inoperable. Moreover, such IMDs that deliver energy to tissue deplete a discrete bolus of battery energy with each delivery typically change operating mode and provide a signal when battery energy depletes to a level that precedes and predicts battery end of life.
Since it is often extremely critical for patients' well-being that IMDs do not cease operating, it is common for IMDs to monitor the level of battery depletion and to provide some indication when the depletion reaches a level at which the battery should be replaced.
Single chamber, dual chamber and right and left heart cardiac pacemaker IPGs and certain implantable cardioverter/defibrillators (ICDs) having a pacing function (herein pacing IPGs) deliver pacing pulses through pacing leads to unipolar or bipolar electrodes disposed about the heart to evoke a depolarization of a heart chamber to establish or regulate the heart rate and rhythm of the heart chambers, typically in a rate response mode wherein the rate is dependent on sensed need for cardiac output. Atrial and ventricular pacing IPGs typically have separate sense amplifiers for atrial and/or ventricular sensing coupled to unipolar or bipolar electrode pairs through atrial and/or ventricular leads. Atrial sense amplifiers detect the presence of intrinsic atrial cardiac depolarization signals, particularly the P-wave, through such unipolar or bipolar electrode pairs and generate an atrial sense event. Ventricular sense amplifiers detect the presence of intrinsic ventricular cardiac depolarization signals, particularly the R-wave, through such unipolar or bipolar electrode pairs and generate a ventricular sense event. Timing and control circuitry and operating algorithms respond to such atrial and/or ventricular sense events to restart a pacing escape interval being timed out that effectively inhibits the delivery of a pacing pulse to the corresponding chamber.
Thus, such pacing IPGs typically operate in a defined or programmed pacing mode to pace and sense in programmed pacing channels, and each delivered pacing pulse consumes battery energy. Typically, the operating system monitors battery energy and depletion and develops an “elective replacement indicator” (ERI) when the battery depletion reaches a level such that replacement will soon be needed to avoid further depletion to a battery “end of life” (EOL) condition. Operating circuitry in the pacing IPG typically responds to issuance of an ERI by switching or deactivating operating modes to lower power consumption in order to maximize the ERI-to-EOL interval. For example, internal diagnostic functions and advanced rate-response functions may be discontinued upon issuance of ERI. Moreover, the battery impedance, voltage and other indicators of the level of battery depletion can be interrogated during a telemetry session and uplink telemetry transmitted for display and analysis employing the programmer as described above.
In addition, the minimum pacing pulse energy that evokes a depolarization or “captures” the heart and the pacing pulse energy level at which loss of capture (LOC) occurs is determined upon initial implantation of the pacing IPG and leads. The physician programs the initial pacing pulse energy (pulse width and/or amplitude) to an energy level exceeding the LOC energy level and providing a “safety margin”. Thus, the pacing pulse energy can be programmed to safely minimize the power consumption to prolong battery life. This testing and programming of the pacing pulse energy level is conducted for each atrial and/or ventricular pacing channel employing an external programmer and telemetry system to sequentially adjust the pacing pulse energy as well as external equipment that can verify that the delivered pacing pulse at the programmed energy has or has not captured the heart.
Many current pacing IPGs also periodically enter a test mode to automatically determine the stimulation threshold employing the sense amplifier of the pacing channel to detect the evoked response. In certain cases, the LOC energy level is determined, and the pacing pulse energy is automatically adjusted to a level providing the safety margin to conserve battery energy. Detection of changes in the stimulation threshold may also be useful for ascertaining the physiological effect of drugs or for diagnosing abnormal cardiac conditions. However, it is more difficult to employ the sense amplifier coupled to the pace/sense electrodes to detect an evoked response or absence of an evoked response for a number of reasons.
First of all, if an evoked response occurs, it does so a short time after delivery of the pacing pulse. But, the sense amplifiers of the pacing channels must be “blanked” during delivery of the pacing pulses to the pacing channels so that the pacing pulse energy does not damage the sense amplifier circuitry. The evoked response may not be sensed if the sense amplifier is blanked for too long a time after the delivery of the pacing pulse.
Secondly, the generation and delivery of a pacing pulse gives rise to the storage of charge in body tissues at the electrode-tissue interface. Such stimulation polarization artifacts or signals, referred to as “after potentials” or “repolarization signals”, typically have much larger amplitudes than those arising from an intrinsic heartbeat. And, if the pacing pulse causes an evoked response in the cardiac tissue, then the evoked response is itself superimposed atop the typically much larger amplitude polarization signal. As a result, conventional pacing IPGs either cannot differentiate, or have difficulty differentiating, between polarization signals and evoked response signals. This problem is further complicated and exacerbated by the fact that residual polarization signals typically have high amplitudes even when evoked response signals do occur. Consequently, it becomes difficult, if not impossible to detect an evoked response using a conventional sense amplifier that uses linear frequency filtering techniques. As a result, most pacing IPGs have great difficulty differentiating between polarization and evoked response signals.
Thus, most pacing IPGs employ sensing and timing circuits that attempt to detect evoked response signals only when the polarization signal is no longer present or has subsided to some minimal amplitude level. Only then is sensing considered reliable. In respect to capture detection methods, however, such sensing may occur only after a significant period of time has elapsed. As a result, many pacing IPGs may not detect evoked response signals with any degree of confidence.
A unipolar pacing lead disposes a single active pace/sense electrode typically affixed at a site of a heart chamber or in a coronary vessel and employs the IPG conductive housing or “can” as a remote return of indifferent pace/sense electrode. A bipolar pacing lead disposes the active and indifferent pace/sense electrodes relatively closely spaced at the site. Typical bipolar leads support the active pace/sense electrode at the lead body distal end as a tip electrode and the indifferent pace/sense electrode proximally from the distal end as a ring electrode. There are typically two electrode-tissue interfaces in a pacing channel, one at the tip electrode, and one at the ring (or casing) electrode that store energy that dissipates after the pace event, creating the after-potential.
The impedance or load presented to the sense amplifier in the pacing channel comprises the impedance of the lead itself, the electrode-tissue interface impedances, and the impedance of the body tissue bulk. The impedances of the body tissue and the lead may be modeled as a simple series resistance, leaving the electrode-tissue interfaces as the reactive energy absorbing/discharging elements of the total load. The tip electrode is the primary after-potential storage element in comparison to the IPG can and ring electrodes. In a pacing channel, a ring electrode typically stores more energy than does an IPG can electrode because the surface area of the ring electrode is smaller that the surface area of the IPG can electrode.
Several methods have been proposed in the prior art for improving the ability of a pacing IPG sense amplifier to detect and measure evoked responses.
For example, U.S. Pat. No. 5,172,690 to Nappholz et al., proposes a tri-phasic stimulation waveform consisting of pre-charge, stimulus, and post-charge segments. The duration of the precharge segment is varied until the amplitude of the stimulation artifact is smaller than the evoked response.
U.S. Pat. No. 5,431,693 to Schroeppel discloses a pacemaker that low-pass filters a sensed signal to remove noise and passes frequencies characteristic of the evoked cardiac signal. The filtered signal is processed to render a waveform signal representing the second derivative of the filtered signal. The second derivative filtered signal is further analyzed to detect minimum and maximum amplitude excursions during selected first and second time windows. The amplitude differences measured during the two time windows are compared to one another to determine whether capture has occurred.
U.S. Pat. No. 5,571,144 to Schroeppel discloses a capture detection system involving analysis of post-stimulus signal morphology.
U.S. Pat. No. 4,114,627 to Lewyn et al. discloses pacing circuitry that delivers pacing pulses through an output coupling capacitor. During delivery of a pacing pulse, the sense amplifier is uncoupled from the cardiac electrode. When the pacing pulse terminates, the output coupling capacitor is coupled to ground through a discharge resistor, thereby discharging electrode polarization.
German Patent No. 4,444,144 to Hauptmann discloses a pacemaker having a sensing circuit that records intracardiac heart signals. An adaptive non-linear noise filter transforms those signals. A matched filter correlates the transformed signals to a pulse pattern and creates an output indicative of heart signals. The sensing circuit reduces faulty signal detection caused by noise filtering by permitting external noise to be distinguished from noise associated with true heart signals.
U.S. Pat. No. 4,549,548 to Wittkampf et al. describes a system whereby the polarity of the lead can switch automatically between unipolar and bipolar configurations to optimize pacing and sensing operations. In one mode, the system employs bipolar QRS sensing and unipolar pacing and T-wave sensing. In another mode, the system employs bipolar QRS sensing and pacing, and unipolar T-wave sensing.
U.S. Pat. No. 5,306,292 to Lindegren describes an autocapture system that automatically tests a number of conductive surfaces to provide the lowest stimulation threshold. In order to reduce the energy consumption, an autocapture unit is provided which automatically tests a number of possible combinations of conductive surfaces for stimulation and selects the combination providing the lowest stimulation threshold for connection to the pulse generator.
U.S. Pat. No. 5,350,410 to Kleks et al. discloses an autocapture system in which one embodiment tests all electrode combinations available to determine the optimum sensing configuration. The electrical post-stimulus signal of the heart following delivery of a stimulation pulse is compared to a polarization template, determined during a capture verification test. A prescribed difference between the polarization template and the post-stimulus signal indicates capture has occurred. Otherwise, LOC is presumed, and a loss-of-P-capture routine is invoked that increases the energy a prescribed amount to obtain capture.
U.S. Pat. No. 5,902,325 to Condie et al. describes a method of using cardiac impedance waveforms to determine cardiac capture resulting from pacing. Circuitry is provided in a pacemaker for obtaining a signal reflecting cardiac impedance, which is known to reliably reflect certain aspects of cardiac function. Circuitry is also provided for monitoring the cardiac impedance waveform during a predetermined capture detect window following delivery of pacing pulses. In one embodiment, the control values against which impedance waveform characterization values are compared are obtained by delivering a series of stimulation pulses having sufficient energy to ensure that capture is achieved, and by monitoring the impedance waveform during delivery of these pulses.
U.S. Pat. Nos. 6,134,473 and 6,144,881, both issued to Hemming et al., describe the Capture Management algorithm implemented, for example, in the Medtronic® Kappa® 700 pacemaker IPGs. The polarity of the positive or negative change in voltage with respect to time (or dv/dt) of the waveform incident on the lead electrodes is monitored during a short period of time immediately following a paced event. In one embodiment, sensing of the evoked response is based upon a relationship between a maximum magnitude of a derivative of a sensed signal and a predetermined threshold reference value. The evoked response is declared when the maximum amplitude of the derivative of the sensed signal equals or exceeds the threshold reference value.
The Capture Management algorithm is periodically run, e.g., once a day at a prescribed time to perform a pacing threshold search (PTS) wherein the pacing pulse amplitudes and pulse widths of the pacing pulses delivered in each pacing channel are incrementally adjusted within a predetermined range to determine the pacing threshold, and the threshold data is stored for analysis of long term trends. When an “Adaptive” mode of the Capture Management algorithm is programmed, the Capture Management algorithm automatically adjusts the pacing pulse amplitude and/or pulse width setting to ensure capture at minimum pacing energy while maintaining the programmed safety margin(s).
The Capture Management PTS was tested in pacing systems configured for bipolar pacing and sensing employing bipolar leads (bipolar polarity) and systems configured for unipolar pacing and sensing employing unipolar leads (unipolar polarity), and the resulting data were presented in Table 3 of the above-referenced '881 patent. The data shows further that capture detection accuracy was enhanced when tip-to-can (herein, the unipolar sense vector) sensing configurations were employed instead of tip-to-ring (herein, the bipolar sense vector) sensing configurations were employed, regardless of pacing pulse polarity.
Bipolar pacing and ICD leads that use short tip-to-ring spacing tend to be more susceptible to evoked response undersensing. In a bipolar pacing and sensing configuration, the evoked response signal occurs sooner after a delivery of a pacing pulse and is more likely to fall within the post-pace blanking period of the sense amplifier due to the relatively short tip-to-ring spacing. In a unipolar pacing and sensing configuration, the tip-to-can spacing is wide, and the occurrence of the evoked response takes place later in time after the pacing pulse is delivered than it does when delivered between the bipolar tip and can electrodes. It has also been observed in practice that Capture Management, as implemented in the Medtronic® Kappa® 700 pacemaker IPGs, occasionally displayed incidents of undersensing of a ventricular evoked response during a PTS leading to a mistaken declaration of a “high output” alert to inform the physician that intervention is necessary. Undersensing of an evoked response was determined to be due to low signal levels present in the programmed sensing configuration. Such undersensing has caused the IPG algorithm to respond by automatically setting the pacing pulse width and amplitude to their maximum programmable values. This false negative response has unnecessarily imposed excessive current drain on the battery and shortened battery life in affected pacing IPGs. Physicians have noted this during patient follow-up and have been able to reprogram the pacing pulse energy and sensing configuration to reduce the likelihood of recurrence. Reprogramming from the bipolar configuration to a unipolar configuration usually resolves the issue when undersensing occurs in the bipolar sensing polarity.
In spite of these advances, improvements are needed in the ability to reliably and consistently detect the evoked response during performance of a PTS and accurately adjust the pacing pulse energy only when necessary.